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Posts Made By: Alan French

November 27, 2008 11:07 PM Forum: Refractors

Star test images for a new (used) Orion ED80

Posted By Alan French
The main problem is undercorrection. Note the brighter outer ring inside focus compared to outside. The green filter shots just inside and outside show a touch of astigmatism.

How far away was the "star?"

Clear skies, Alan

November 30, 2008 02:45 PM Forum: Telescope Making

Over-Under Refractor Binoscope

Posted By Alan French
That always seemed like a good approach (and thanks for the example Thomas!). You only need two standard star diagonals, so it eliminates a lot of glass.

I suspect you'd have to design it for your IPD. A little rotation of the pair could vary this a bit, but then the exit pupil height would differ between the two.

Likely not user friendly during the day except for large distances. The views would be displaced vertically for close objects. With normal binoculars and close objects you get two images displaced horizontally, which gives you something close to the typical "movie view" through binoculars - two partially overlapping views displaced horizontally (nicely eliminated by the objective set up on the Pentax Papilio). Your eyes can obviously deal with this, but I think vertical displacement would be a impossible to handle.

Clear skies, Alan

December 20, 2008 08:12 PM Forum: Refractors

Triplet Semi-Apo?

Posted By Alan French
Kevin Brennan said:

I am curious if anybody is familiar with the type of scope that is in auctions right now. It is large refractor listed as a "semi-apo" and every time I hear that term it makes me cringe. Does anybody know if these designs had an ED element. If not, how does the third element work exactly?

Clear skies,
Kevin

For a modern apochromat of reasonable focal length and superb color correction the obvious candidates are fluorite or a fluor-crown like FPL-53. Either is an expensive choice, and availability is a problem is you are an individual making a single lens. An alternative is to use a slightly abnormal glass to produce a lens with better color correction than a traditional achromat, but still has some secondary color. The first "super-planetary" lens Roland Christen showed at Stellafane, a 5" f/12 triplet, used BaFN-10 and KzFSN-4 to synthesize a flint element with the desired optical properties, and combined it with a BaK-1 crown to produce a well corrected lens.

Back before the days of modern apochromats, lunar and planetary observers would have been quite happy with an 8"f/15 achromat. Of course, such an instrument is long and difficult to mount. A modern 8" f/9 with superior color correction and a superb figure sounds very attractive. It certainly tempts me, but I have enough telescopes.

Clear skies, Alan

December 20, 2008 11:16 PM Forum: Global Warming - REAL or NOT

Significant Evidence of GW

Posted By Alan French
Rod Kaufman said:

[URL snipped]
How bad does it have to get for some people?...

Some people will never admit to a mistake. There are prosecutors who refuse to accept that they convicted the wrong person in spite of overwhelming evidence they did.

Highly recommended reading...
http://www.mistakesweremadebutnotbyme.com/

Clear skies, Alan

December 21, 2008 12:43 AM Forum: Refractors

Triplet Semi-Apo?

Posted By Alan French
I got curious about prices in this aperture realm...

D&G 8" f/15 achromat $3695
TEC 180mm f/7 fluorite oiled triplet $19,000
APM 203mm f/9 triplet $26,695

Prices are for OTAs, and may include a few accessories.

Clear skies, Alan

January 23, 2009 07:45 PM Forum: Solar System Observing

Venus in Daylight

Posted By Alan French
Scott Mortimer said:

Wow, so you were able to see it naked eye as well?? that IS cool, I've never done that, will have to give it a try-

Scott

Scott,

If the skies are clear, it is not hard to do. Find out when Venus transits and is highest in the sky from your location and note the altitude. Stand to the east of a building in the shade near the transit time, and just scan the southern sky at about the right altitude. I find I always underestimate how high to look.

One problem I have is my eye's apparent inability to focus at infinity when looking at a blue sky. Often a few clouds or a passing plan can be helpful here and make it easier to spot Venus.

If you can't find it by eye alone, search it out with binoculars first. Once you find it, I think you'll be surprised at how easy it is to see, and wonder why you didn't spot it a little sooner.

Clear skies, Alan

January 24, 2009 11:27 PM Forum: Solar System Observing

Venus in Daylight

Posted By Alan French
I found it just after 4:00 PM today without any trouble. It was also pretty easy to spot from inside through one of the south facing windows.

Clear skies, Alan

January 26, 2009 01:37 AM Forum: TeleVue

The Reality of Planetary Amateur Telescopes

Posted By Alan French
Amateurs will argue about the benefits of aperture forever, but I have concluded, based on my experience, that aperture is a valuable attribute for planetary observing. My first real telescope was an 8" f/8 Newtonian with a superb mirror, and it provided planetary views I have never seen matched in any 4" refractor.

The article's author complains about the brightness with larger apertures. This makes me think of some of Harold Hill's comments in his "A Portfolio of Luna Drawings," where he opined that the eye/brain has to get used to a certain brightness level, and it takes time to be able to wring out all that a new aperture will show.

I spent several nights with my wife's 14.5" f/6 when Saturn were well placed. I used it with and without a 6.5" off-axis mask. Although it was most certainly seeing limited at full aperture, the better color rendition and larger exit pupil (at the same magnification) were appreciated, and there were often moments when more detail was visible. The mask went in the trash.

Clear skies, Alan

January 26, 2009 03:59 PM Forum: TeleVue

The Reality of Planetary Amateur Telescopes

Posted By Alan French
david goldman said:

long f/ratios improves contrast and definition more than anything else.

Why?

Clear skies, Alan



January 30, 2009 01:30 AM Forum: Solar System Observing

Venus and the Moon, 2009/1/29

Posted By Alan French
Dave,

Nice. We had a fortuitous clearing provide a nice view of the conjunction. Then the clouds rolled in again.

Clear skies, Alan

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